To realize a balun transformer within a very wideband frequency range, it should be noted that simple classic L-C baluns can not cover this frequency band.
There are two ways to realize it.
1-Using a transformer which has tight coupling coefficient and one of the output pins is grounded. In this way a well designed transformer will behave a balun.
But transformer losses and especially low frequency response must be optimized. If you increase inductance value of primer side to hold low frequency behaviour , self resonance frequency of the primer inductance will drop in frequency of interest.( sure)
2- Using ferrite double aperture transformer. In this method losses will be higher than but low frequency response might be better controlled.
In my opinion using Epcos( formerly siemens-matsuhita ) transformers is better. Because to find ferrite with the same characteristic and production within a same relibility is much difficult.
In this way , there are some restrictions such as frequency response may not be flat as desired and impedance ratios can change by frequency.
Indeed design a balun transformer that will work in very wide band frequency range is really difficult.